The present invention generally relates to systems for setting up sensor/encoder pairs and more particularly relates to a method and a device for checking or determining the installation air slot between an active sensor and an encoder.
Sensors of this type are used as motion sensors, for example, for sensing the rotational behavior of a vehicle wheel (wheel speed sensors) in slip-controlled brakes or anti-lock systems. The rotational movement is executed by a pole wheel provided in the encoder and producing the rotating magnetic field. This magnetic field is sensed by the sensor having a pick-up for measuring data which is for example a magneto resistive bridge (AMR bridge) that responds to variations in the magnetic field.
In order to ensure trouble-free transmission of the movements from the encoder to the sensor, care must be taken that the air slot (i.e. space between the sensor and the poles of the pole wheel) will never exceed the maximum air slot. In addition, there must be a defined minimum difference between the actual air slot and the maximum air slot in order that the maximum air slot will not be exceeded, not even temporarily, due to tolerances of structural elements, variations in temperature and dynamic deformations, or a certain variation in the supply voltage. To compensate for the changes in the air slot which are caused by the above variations, the sensor includes a trigger stage which is connected downstream of the magneto resistive bridge and by which the sensor output signal is maintained constant.
However, the arrangement set forth above suffers from the shortcoming that it cannot be checked after the assembly of the sensor whether the installation air slot is sufficiently smaller than the maximum air slot, i.e., whether a safety minimum difference exists between the two slots. The fact that the sensor output signal is present only implies that the air slot does not exceed the maximum air slot in that instant. However, the air slot may actually be already so large that the maximum air slot is nevertheless exceeded, e.g. temporarily, due to an unfavorable combination of the above-mentioned tolerance conditions, so that the transmission between encoder and sensor may at least be disturbed.
Measuring the actual distance by means of the signals received from the sensor is not possible due to the trigger stage installed in the sensor because this trigger stage generates a constant sensor output signal as long as the air slot does not exceed the maximum air slot.
In view of the above, an object of the present invention is to seek for a possibility of checking or determining the actual installation air slot and, especially, the difference between it and the maximum air slot in a sensor of the type mentioned hereinabove.
This object is achieved by a method of the type referred to hereinabove which is characterized by the following method steps:
reducing the supply voltage of the sensor to at least one value by which the maximum air slot is each time decreased by a desired test measure, and
comparing the at least one test measure with a desired difference between the maximum air slot and the existing air slot by determining a variation of the sensor output signal by the reduction of the supply voltage, as well as evaluating the comparison result by signaling and/or storing.
This object is further achieved by a device of the above-mentioned type which is characterized by
a first means which permits reducing the supply voltage of the sensor to at least one value by which the maximum air slot is decreased each time by a desired test measure, and
a second means for comparing the at least one test measure with a desired difference between the maximum air slot and the actual air slot by sensing a variation of the sensor output signal by the reduced supply voltage as well as for evaluating the comparison result by signaling and/or storing.